This invention relates to heat exchangers in general and more particularly concerns refrigerator evaporators having a spread or flared serpentine configuration wherein adjacent straight runs of the tubing are not parallel.
Household refrigerators typically operate on the simple vapor compression cycle. Such a cycle includes a compressor, a condenser, an expansion device, and an evaporator connected in series and charged with a refrigerant. The evaporator is a specific type of heat exchanger which transfers heat from air passing over the evaporator to refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is used to refrigerate one or more refrigerator compartments. A refrigerator evaporator for vapor compression cycles must satisfy several functional requirements. The most important of these are discussed below.
First, an evaporator must be thermally effective; that is, refrigerant flowing within the evaporator should exist at a saturation temperature which is not much lower than the temperature of the air flowing over the evaporator. This is because the higher the saturation temperature of the refrigerant is, the greater the efficiency of the refrigeration cycle will be. Consequently, the evaporator exit saturation temperature is indicative of the thermal effectiveness of the system.
Next, the evaporator must be effective in dehumidifying the air being refrigerated. If the air is not sufficiently dehumidified, then water droplets will form on various surfaces in the fresh food compartment and ice will form on various surfaces in the freezer compartment. An evaporator dehumidifies air by condensing air moisture as the air passes over the evaporator. Thus, air bypass of the evaporator must be minimized for effective dehumidification.
Because condensed moisture forms as frost or ice on the leading side of the evaporator surfaces, an effective evaporator must also be able to accumulate large amounts of ice without producing substantial air blockage. Ice accumulation on evaporator surfaces will eventually cause substantial cycle efficiency degradation. Most refrigerators are thus provided with a resistance heater which is periodically activated to defrost the evaporator. Defrosting energy use is parasitical. Frequent defrosting results in greater system energy use because much of the defrost heat is unavoidably diverted to un-iced surfaces rather than just melting ice. A desirable evaporator is therefore one which can accumulate substantial amounts of ice before defrosting is required.
Lastly, the evaporator must be compact because it occupies refrigerated space. The larger the evaporator and its associated components (i.e., fan, defrost heater, ducting, etc.) are, the less food storage space is available for the same cabinet size and energy use.
FIGS. 1A and 1B show a conventional top mount refrigerator 10 including an outer cabinet 11 containing a freezer compartment 12 and a fresh food compartment 14 with the freezer compartment 12 being disposed above the fresh food compartment 14. The freezer compartment 12 is maintained at below freezing temperatures and the fresh food compartment 14 is maintained at food preserving temperatures (i.e., temperatures which are typically above freezing but low enough to preserve food) by circulating air over a conventional evaporator 15. The evaporator 15 is a length of tubing having a plurality of elongated straight tube segments 22 and a plurality of return bent tube segments 24 formed in a serpentine coil arrangement. The straight tube segments 22 are connected in series for refrigerant flow therethrough, with the outlet of each straight tube segment 22 connected to the inlet of the next straight tube segment 22 by a bent tube segment 24. The straight tube segments 22 are generally closely spaced and parallel to one another. Some of the bent tube segments are bent in a different plane to define additional rows of the straight tube segments (such as the three rows shown in FIG. 1A). The evaporator tubing is typically provided with fins to improve the heat exchange per unit length of the tube.
The evaporator 15 is disposed within an evaporator chamber 16 located in the freezer compartment 12; the evaporator 15 extending lengthwise across the freezer compartment 12. The evaporator chamber 16 is formed by a vertically extending front panel 17 separating the evaporator chamber 16 from the rest of freezer compartment 12, substantially parallel side walls 18, and an inner rear liner 19 of the refrigerator 10. Thus, the evaporator chamber 16 fills an entire top-to-bottom portion in the rear of the freezer compartment 12. The evaporator 15 is supported within the evaporator chamber 16 by each of the bent tube segments 24 being retained within a corresponding opening in one of a pair of substantially parallel end channels attached to respective ones of the side walls 18. Each of the end channels may be formed by galvanized steel or aluminum. The end channels may have mounting holes therein for mounting to the freezer liner of the refrigerator 10. The liner may have special receptacles or supports to allow for attachment.
A motor driven axial fan 20 is positioned in the upper portion of the evaporator chamber 16 so that the rotational axis of the fan is perpendicular to the front panel 17. The fan 20 causes cooled air to be discharged through openings 21 in the front panel 17 of the evaporator chamber 16 into the freezer compartment 12. Some of the air flows through a passage (not shown) into the fresh food compartment 14 in a manner well known in the art. This division of the cooling air is such that the freezer compartment 12 is maintained at below freezing temperatures and the fresh food compartment 14 is maintained at food preserving temperatures. The fan 20 also draws air from the freezer compartment 12 and the fresh food compartment 14 into the lower portion of the evaporator chamber 16. Thus, the return air flows vertically over of the evaporator 15 in a transverse manner. This transverse air flow is substantially perpendicular to the elongated straight tube segments 22 of the evaporator 15. After flowing over the evaporator 15, the direction of air flow is first bent 90.degree. to the left by the fan 20 and is then bent 180.degree. to the right to flow through the openings 21. Thus, the air flow must be bent through a total angle of 270.degree. in passing over the evaporator 15 and through the openings 21.
This arrangement requires substantial space for inlet and exit plenums and incurs inlet and exhaust pressure drop which provides no heat transfer benefit. As seen in FIG. 1A, the plenum and fan space uses almost as much of the evaporator chamber volume as the evaporator 15. Furthermore, frost tends to build up only on the lowermost portion of the evaporator 15. This is because as the moist return air initially contacts the bottom of the evaporator 15, most or all of the moisture is immediately removed from the air and deposited as frost on the lowest portion of the evaporator 15. As the air continues to pass over the evaporator 15, there is little or no moisture remaining in the air so that the upper portion of the evaporator 15 receives little or no frost. This uneven frost distribution means that frost tends to build up quickly and frequent defrosting is required.
Accordingly, there is a need for an energy efficient, compact refrigerator evaporator which can accumulate large amounts of frost.